1. Technical Field
The invention relates to the art of on-board detection systems for catalyst efficiency, and more particularly to the art of relating such detector systems to engine controls that influence catalyst efficiency.
2. Discussion of the Prior Art
One of the important concepts of the 1970's was the closed loop air/fuel ratio (A/F) control for engines. Such control analyzed the oxygen content of the exhaust gases and used the analysis information to modify the A/F to bring it into conformity with a desired narrow range (A/F window) that maximizes catalyst converter efficiency. The sensor is placed upstream but close to the catalytic converter, and the A/F comparator computer, ingested with background information, is used to change the A/F controller on a continuous basis providing an overall average control of A/F near stoichiometry (see U.S. Pat. No. 4,000,614).
Such feedback control has been amplified by the use of two exhaust gas oxygen sensors, one upstream of the catalyst and the other downstream of the catalyst (see U.S. Pat. No. 3,939,654). The information of both sensors is integrated with the hope of more accurately maintaining the A/F within such desired window. Unfortunately, the exhaust gas oxygen sensors undergo a switching function at stoichiometry (i.e., from rich to lean or lean to rich), and due to the time delay between signals of the sensors, there is considerable hunting and poor response of the feedback engine control system. The prior art has resorted to extremely complex jumpback software algorithms to compensate for the signal delay between the sensors (see U.S. Pat. No. 4,761,950). However, the use of upstream and downstream exhaust gas oxygen sensors about a catalyst has not led to ideal catalyst efficiency levels and has not prevented catalyst degradation.
The art has recognized that the upstream and downstream exhaust gas oxygen sensor signals will be different resulting from oxygen storage capabilities of the catalyst (see U.S. Pat. Nos. 4,622,809; 4,007,589; and GB 2,178,857). Although the prior art has made this observation, it has not been able to devise a system that provides an accurate determination of good and bad catalysts even though armed with this information. One reason for this inability is that oxygen storage is not only caused by the noble metal (the key elements that determine catalyst life) but also by stabilizing oxide coatings on the substrate such as cerium oxide. In fact, the oxide coatings may account for the major proportion of oxygen storage, which coatings do not provide for the essential detoxification conversion. Thus, in an engine-catalyst control-loop having integrated upstream and downstream oxygen sensors, the cerium oxide coating may become evaporated due to over-temperature operation, or may be changed in crystal structure by phosphorus or silicon poisoning. The dual sensors will provide an indication that the catalyst is bad when in fact the noble metals may still be functioning properly. This "bad" indication results from the ability to see only large gross differences in oxygen storage, which differences are heavily weighted to the cerium oxide function, and also to the fact that noble metals inherently cannot store oxygen very long thereby making oxygen storage a very fleeting measure of functionality.
What is needed is a detector system that can differentiate accurately the oxygen storage capability of the catalyst due to the noble metals as opposed to that of oxide coatings. Such system must be able to magnify or accelerate such inherent oxygen storage capability of precious metals to make it more readily detectable.